Am I allowed to travel with the kind of D visa "mult" even by car or bus?

A visa gives you permission to enter a respective land. It does not limit you in travel means - you can cross the border by plane, by car, by bus, by ship, even walking.

A German D visa allows you to stay in Germany for the duration of the Visa, and to travel to all other Schengen states as if you had a C visa. Multiple entry means that you can enter the Schengen area multiple times. Within the Schengen area, you can cross borders as many times as you want, even with a single entry visa. There are places where crossing borders is as easy as crossing to the other side of the street.

• If you want to drive through non-Schengen countries, you will need visa for them unless they allow you visa-free entry. If you fly directly from your homeland to a Schengen country, all you need is a Schengen visa.
• You might have heard that some Schengen nations reintroduced border controls, especially at their land borders. What that means is that papers are being checked again, so you should have your passport and visa along when you cross internal Schenged borders. (Normally there are no checks, but spot checks can happen near the border.) Your papers get stamped only at the external Schengen border.
• Some Schengen countries and neighbouring countries along the Balkan Route have reinforced border controls on northward travel. Again they should let people with valid visa through, but some border crossing points got closed.
• You have received for a student visa and you are expected use it to study, not spend all your time traveling. But there are no lectures on Christmas, so travel around the holidays is no problem in that regard.

Your type D visa grants you permission to study in germany. It also grants you permission to visit and transit through other schengen countries under the 90/180 rule (time in germany on your D visa doesn't count against the 90/180 rule). From this point of view it doesn't matter whether you enter by land or air.

The big difference between land transport and air transport is.

When you fly over a country without landing you are not considered to be "entering" that country from an immigration point of view. Rules vary for transiting without leaving the airport but in most countries (north america being an exception) the rules are much easier for airside transits than for people entering the country.

When you travel by land you have to enter every country along your route. Within the schengen area you are fine as mentioned above but once you move beyond the Schengen area you need to carefully check the Visa policy of each country along your route.

Furthermore if you are travelling by car there may be issues with the car itself. In addition to checking visa requirements you would also need to check policies on temporary import of vehicles.